Abstract It is an extraordinary accomplishment that most developing neuronal networks achieve an appropriate level of excitability, during a dynamic period of embryonic development when there are several challenges to a network's excitability. Therefore, it is not surprising that the incidence of network hyperexcitability is higher in the neonatal period than in any other age group. Errors in such a complicated process can lead to alterations in the excitability of neonatal spinal circuit, which can be observed behaviorally as myoclonus, hypertonia, recurrent tremor, and spasticity. Understanding the rules and mechanisms that underlie the maturation of network excitability are therefore essential. Recently, an exciting new field has emerged that provides critical insights to understanding the rules that networks follow in order to achieve appropriate levels of activity. Many studies have now shown that networks homeostatically maintain activity levels within an appropriate range by adjusting synaptic strength (homeostatic synaptic plasticity). The vast majority of these studies have blocked network activity in vitro (culture systems) for days, and changes in synaptic strength are in a compensatory direction. Compensatory changes in intrinsic cellular excitability (cell's responsiveness to synaptic input) also likely contribute to the homeostatic process, although these changes have received far less attention than synaptic compensations. We have found that changes in cellular excitability mediate the initial homeostatic recovery of perturbed activity levels in the embryonic spinal cord. The objective of this application is to better understand the role and mechanisms underlying homeostatic changes in cellular excitability and synaptic strength in the developing circuit. We are proposing to perturb network activity in a more realistic manner in the living embryo, allow for the homeostatic recovery of activity, and then carry out a comprehensive assessment of the proteins that mediate the initiation, signaling, and expression of compensatory changes in excitability and synaptic strength. The project will provide a more extensive, realistic understanding of homeostatic plasticity, and define its role in the maturation of network excitability. Further, the study will identify proteins underlying each form of homeostatic plasticity, and therefore provide therapeutic targets for conditions of hyperexcitability. Our approach introduces a new method into the homeostatic field that will elucidate a molecular network that will identify genes that associate with human disease, and help us better understand the function of homeostatic plasticity.